The present invention relates to electronic signal amplifiers and, more particularly, to electronic signal amplifiers which have altered gain characteristics over the range of input electrical signals provided thereto.
Electronic signal amplifiers are used in many kinds of electronic circuit systems. The basic capability of such electronic signal amplifiers is to provide an output electrical signal therefrom which is an amplified version of the input electrical signal provided thereto at least over selected frequency and amplitude ranges of that input signal. However, typically at some points in these ranges, such amplifiers become unable to provide an output signal that is substantially an amplified reproduction of the input signal because of limitations in the amplifier performance capabilities which may be either desired or undesired. Thus, there will be a magnitude of the input signal for any practical amplifier in excess of which that amplifier will no longer be able to provide an output signal of a magnitude maintaining the same proportion to the input signal magnitude that it does at lower values of the input signal. That is, the output signal can no longer increase in magnitude to proportionately match a corresponding increase in the input signal magnitude.
If there are no special provisions in the design of the amplifier for this situation and the change in proportionality is quite substantial over a very small magnitude range of the input signal, the output signal is said to be "clipped" in that the peak values of that signal are forced by the amplifier to not exceed some internally determined clipping limit. Such a result greatly distorts the reproduced output signal in comparison with the input signal which, for instance, gives a very undesirable result if the amplifier is amplifying acoustically derived electronic signals in a system intended to provide an audio output to a listener. If the clipping level is relatively great compared to the magnitude range containing the great majority of the input signal magnitudes, there will be relatively little distortion as only an occasional peak will be clipped. On the other hand, if the clipping limit is relatively low compared to this predominant range of signal magnitudes encountered in providing the audio output, the resulting distortion will be quite irritating to the listener.
A provision can be made in the amplifier design, usually termed automatic gain control or automatic volume control, which serves to "compress" the output signal by reducing the gain of the amplifier whenever the output signal becomes too great. Such an arrangement can avoid much of the distortion that would come from clipping, and can provide a further value in limiting the magnitude of output signals which would be objectionable in magnitude even if undistorted. That is, in an audio system, for instance, the output sound can be too great for the comfort of the listener, i.e. too loud, even though undistorted. Hence, such automatic gain control will provide a substantially better listening experience for that listener.
The rapidity at which such an automatic gain control system can react to the occurrence of unusually large signal magnitudes at the output of the amplifier, both the time duration in which gain changes are initiated and the time duration in which gain changes are terminated, can cause different effects in the subsequent signal portions. Quick implementation of gain changes allows affecting the spoken syllables in an audio signal while slow implementation of gain changes allows the system to respond to the average sound background to thereby control an average of the magnitude, or the loudness, of an audio output signal. Quick response is needed in an automatic gain control system if moments of excessive magnitude in the output signal are to be avoided, and yet relatively slow gain changes are needed to suppress elevated levels of background noise in the audio output signal. Differing times of termination of gain changes can lead to the introduction of unwanted audio artifacts into the output signal. An automatic gain control system which responds both quickly and slowly to the need for gain changes to prevent sudden loudness increases and suppress background noise can often provide an improved audio output signal for a listener.
The sensing of signal peaks and the sensing of average signal levels is much more easily accomplished at the output of the amplifier, where signal magnitudes are relatively large, than it is at the input of the amplifier where the signal amplitudes are relatively small. Thus, automatic gain control subsystems for preventing undesirable peaks and for reducing background noise levels usually are designed to sense the amplifier output signal as a basis for imposing the differing gains selected at various times through determining the magnitudes in that signal which the input signal directs the amplifier to provide in those times.
On the other hand, imposing a continual gain change based on magnitude rather than just upon the occurrences of magnitude peaks, typically done for the purpose of compressing the magnitude range of amplified signals, can then be done more conveniently by sensing the input signal to the amplifier. This input signal is not affected by the gain changes imposed by the automatic gain control system sensing the output signal as describe above. In audio systems such as hearing aids, such compression is often provided because the hearing sensitivity of many listeners with impaired hearing is often quite nonlinear. As sounds become less intense, they can seem to such a listener to become much, much less intense. Alternatively, in some listeners with impaired hearing, louder sounds are perceived as being much, much louder than normal. Thus, the wearer of a hearing aid with this compression capability will have the magnitude range of the incoming signal compressed by some predetermined relationship, and use the volume control to shift the output magnitude range of the compressed signal to that loudness range found most desirable by that listener.
This relationship is usually expressed as a compression ratio which is the slope of the line relating the output signal values in decibels to the input signal value in decibels, a relationship which implies the output signal magnitude, after removal of the logarithmic expressions indicated present by the decibel measure units, is related to the value of the input signal magnitude raised to a fractional power or exponent. A commonly chosen fractional power is the square root of the input signal magnitude which is often stated to be a 2:1 compression ratio.
However, such a compression ratio selection is not necessarily the best selection in all circumstances. Thus, if a compression ratio is selected which is too small in not compressing the incoming signal magnitude range sufficiently, and the volume control is set so that the maximum magnitude sound is just below the discomfort level for the hearing aid user, relatively low intensity sounds may not be heard since such low intensity sound levels will be in the impaired hearing intensity range of such a user. If, alternatively, the signal compression ratio is too great and the volume control is adjusted so that the maximum sound magnitudes are just below the level of discomfort of the user, low intensity sounds will be relatively loud compared to the high intensity sounds so that ordinary background noise will seem relatively loud to the user.
Persons with impaired hearing generally require some amplification of the sounds they wish to hear which is almost always provided by an electronic amplifier having a suitable gain in a hearing aid. On the other hand, everyone, including the hearing impaired, will suffer discomfort if sounds reaching their ears become too loud. Thus, there is a desire for an amplifier that will provide electronic signal gain therethrough but one preventing extreme sound peaks from occurring in its output without the occurrence of clipping in its output signal and its associated distortion, and which compresses the magnitude range in accord with the gain desired by the user such that selecting a higher gain also selects a correspondingly higher effective compression ratio.